1. Field of the Invention
The present invention relates generally to a valve timing control system for an internal combustion engine for adjusting open and close timing of intake and/or exhaust valve depending upon engine driving condition. More specifically, the invention relates to a valve timing control system for adjusting valve open and/or close timing with assurance of smooth thrusting motion of a phase adjusting means.
2. Description of the Background Art
One typical construction of the conventionally known valve timing control system has been illustrated in the U.S. Pat. No. 4,535,731. In the disclosed construction, a camshaft carries a camshaft synchronous rotary member. On the other hand, a timing sprocket is mechanically connected to a crankshaft via a timing chain for rotation in synchronism with engine revolution. An intermediate gear member of generally cylindrical construction is disposed between the timing sprocket and the camshaft synchronous rotary member. The intermediate gear member has helical gear teeth formed on at least one of the inner and outer periphery thereof. The intermediate gear member is axially shiftable by a hydraulic means for causing phase shift between the crankshaft and the camshaft.
In the shown construction, the intermediate gear member is shiftable between a first and initial position and a second shifted position. At the first position of the intermediate gear member, the phase relationship between the camshaft and the crankshaft is maintained at initial phase relationship. When the intermediate gear member is shifted to the shifted position, the phase relationship is varied to advance the valve timing relative to the engine revolution.
Though such prior proposed valve timing control system is successful to improve the engine driving performance in certain aspect, however, performance realized in the prior proposed system was not satisfactorily high, particularly in terms of response characteristics. Namely, the torsional torque exerted on the camshaft cannot be maintained constant. Rather, the rotational torque at the camshaft periodically fluctuates upon opening and closing of the intake valve and the exhaust value. Such fluctuation of the rotational torque on the camshaft will be transmitted to the intermediate gear member. This fluctuating rotational torque can serve as resistance against shifting motion of the intermediate gear member to cause inching motion of the intermediate gear member. This clearly slows down shifting speed of the intermediate gear member to degrade response characteristics in adjusting the phase relationship of the camshaft relative to the engine revolution cycle.